1. Field of the Invention
Generally, the present invention relates to metrology in the manufacturing of microstructures, such as integrated circuits, and, more particularly, to the measurement and monitoring of the dimensions of microstructure features by means of metrology tools, such as a scanning electron microscope (SEM).
2. Description of the Related Art
In manufacturing microstructures, such as integrated circuits, micromechanical devices, opto-electronic components and the like, device features such as circuit elements are typically formed on an appropriate substrate by patterning the surface portions of one or more material layers previously formed on the substrate. Since the dimensions, i.e., the length, width and height of individual features, are steadily decreasing to enhance performance and improve cost-effectiveness, these dimensions have to be maintained within tightly set tolerances in order to guarantee the required functionality of the complete device. Usually a large number of process steps have to be carried out for completing a microstructure, and thus the dimensions of the features during the various manufacturing stages have to be thoroughly monitored to maintain process quality and to avoid further cost-intensive process steps owing to process tools that fail to meet the specifications in an early manufacturing stage. For example, in highly sophisticated CMOS devices, the gate electrode, which may be considered as a polysilicon line formed on a thin gate insulation layer, is an extremely critical feature of a field effect transistor and significantly influences the characteristics thereof. Consequently, the size and shape of the gate electrode has to be precisely controlled to provide the required transistor properties. Thus, great efforts are being made to steadily monitor the dimensions of the gate electrode.
Device features are commonly formed by transferring a specified pattern from a photomask or reticle onto a radiation-sensitive photoresist material by optical imaging systems with subsequent sophisticated resist treating and development procedures to obtain a resist mask having dimensions significantly less than the optical resolution of the imaging system. It is therefore of great importance to precisely control and monitor the dimensions of these resist features, as these features that determine the dimensions of the actual device features may be “reworked” upon detecting a deviation from the process specification.
A frequently used metrology tool for determining feature sizes in a non-destructive manner is the scanning electron microscope (SEM), which is able, due to the short wavelength of the electrons, to resolve device features having dimensions, also referred to as critical dimensions (CD), in the deep sub-micron range. Basically, in using an SEM, electrons emitted from an electron source are focused onto a small spot of the substrate via a beam shaping system. Secondary radiation generated by the incident electrons is then detected and appropriately processed and displayed. Although an SEM exhibits a superior resolution compared to optical measurement tools, the accuracy of the measurement results strongly depends on the capability of correctly adjusting the focus of the SEM, i.e., appropriately adjusting one or more tool parameters, such as the lens current of a magnetic lens, the acceleration voltage of the incident electron beam, and the like. For instance, in scanning a device feature such as a line, an electron beam that is not set to the optimized focus condition may result in an increased measurement value, whereas scanning a trench with a slightly defocused electron beam may lead to an underestimation of the actual trench width. Since the ever-decreasing features sizes of sophisticated microstructures pose very strict constraints on the controllability of critical dimensions, the measurement tolerances of the metrology tools become even more restricted as the tightly set critical dimensions have to be monitored in a reproducible and reliable manner.
In view of the problems outlined above, SEM tools have been recently introduced that are adapted to carry out dimension measurements in a substantially completely automatic manner. That is, these SEM tools repeat for each measurement target a process sequence including pattern recognition, automatically focusing the tool and measuring the pattern under consideration. With shrinking features sizes, however, automatically determining optimum resolution conditions and subsequently determining reliable measurement results by image analysis routines becomes more and more challenging as, for example, the beam shaping system of modern SEM tools is designed to give an optimum resolution with lower and lower focus depth, while at the same time features with steadily reduced sizes produce less signal for the automated focus and image analysis algorithms implemented in these tools. Consequently, if any routine for determining an optimum resolution of an inspection tool is carried out, the obtained setting may include a certain degree of uncertainty that is determined by the specific inspection tool used and the operational behavior, for example the implemented focus-finding and image analysis algorithms, and the current conditions thereof, as the resolution depends on a variety of parameters, such as condenser lens current, stigmatism, working distance, accelerating voltage, and the like.
Thus, although modern state of the art inspection tools allow improved precision and throughput by automatic determination of appropriate focus and resolution conditions in combination with image analysis, the demand for tightly set measurement tolerances required for features sizes of 0.08 nm and even less may not be satisfactorily met by presently available inspection tools. For example, conventional SEM devices may generate CD measurement data on the basis of edges automatically identified in the image, since, due to the so-called edge effect, the emission of secondary electrons is enhanced when the focused electron beam encounters during its scanning movement a protrusion, such as the sidewall of a line feature. The increased release of secondary electrons results in a bright spot on the displayed image due to the increased current produced by the secondary electron detector. Typically, for measuring critical dimensions in resist features and etched features of advanced semiconductor devices, electron energies of several hundred volts up to approximately 2000-3000 volts are used to release electrons close to the sample surface for providing a high contrast caused by edges of the sampled feature, such as a resist line. By determining edges in the image produced by the secondary electron detector, a corresponding distance and thus CD of the feature may be estimated. However, the assessment of the detector image by automated image analysis algorithms strongly depends on the SEM settings, as is previously explained. Moreover, the pronounced edge effect may also give rise to image artifacts, thereby resulting in incorrect CD estimates. The situation may even become worse for advanced devices, such as gate electrode structures of the 90 nm technology and less, since here typically highly sensitive resists may be used which suffer from an increased sensitivity to interaction with the incident electron beam, thereby further contributing to, in addition to image artifacts, a reduced overall resolution. Increasing the number of measurement runs per feature in order to improve measurement accuracy may be less desirable due to an increase of measuring time and higher complexity for evaluation algorithms, as these algorithms may have to account for corrections with respect to the increased interaction.
In view of the above problems, there exists a need for a technique that enables the determination and monitoring of dimensions of features in the deep sub-micron regime with a minimal variation and high statistical significance.